Expression of eukaryotic gene products in prokaryotes is sometimes limited by the presence of codons that are infrequently used in E. coli. Expression of such genes can be enhanced by systematic substitution of the endogenous codons with codons over represented in highly expressed prokaryotic genes (Robinson et al., Nucleic Acids Res. 12:6663, 1984). It is commonly supposed that rare codons cause pausing of the ribosome, which leads to a failure to complete the nascent polypeptide chain and a uncoupling of transcription and translation. Pausing of the ribosome is thought to lead to exposure of the 3' end of the mRNA to cellular ribonucleases.